In modern motor vehicles, the driver is typically assisted by a brake booster during braking. The brake booster makes it possible to reduce an actuating force on the brake of the motor vehicle which is required to achieve the desired braking effect. In passenger vehicles and light commercial vehicles in particular, vacuum brake boosters are generally employed which draw their energy from a vacuum, i.e., from a pressure difference between an atmospheric pressure and a gas pressure prevailing in a vacuum reservoir.
It is noted that within the scope of this description, vacuum is understood to mean the degree of difference between the atmospheric pressure, assumed herein to be 1013 hPa, and the pressure prevailing in the vacuum reservoir, which during operation is typically less or equal to 1013 hPa, i.e., the vacuum is assumed to have a positive sign.
The vacuum prevailing in the vacuum reservoir may be generated in a number of ways. Conventionally, the vacuum is usually tapped at a throttle valve in the intake manifold of an internal combustion engine. The strength of the vacuum so generated depends on the operating state of the internal combustion engine. Alternatively, a mechanical vacuum pump or an electric vacuum pump may generate the required vacuum. The use of an electric vacuum pump in this case has the advantage that the vacuum may be generated regardless of the state of the internal combustion engine. This may be significant, particularly in the case of vehicles which have a so-called start-stop function or in the case of hybrid vehicles. In electric vehicles, the use of an electric vacuum pump is presently the only practical option for generating the vacuum for the brake booster.
However, the characteristics and the timing of the generation of a vacuum in the case of an electric vacuum pump differ from those of mechanical vacuum pumps or of an intake manifold vacuum. A mechanical vacuum pump continuously generates a vacuum and an intake manifold vacuum is built up mainly by applying or releasing a gas pedal of the motor vehicle, whereas an electric vacuum pump is typically activated with the aid of a regulator.
For example, German Published Patent Appln. No.10 2009 046 006 describes inter alia a method for monitoring the function of an electric vacuum pump in a braking system. In this method, the vacuum pump is activated during operation between a switch-on pressure representing a first pressure threshold and a switch-off pressure representing a second pressure threshold. Thus, the electric vacuum pump does not start generating a vacuum for the brake booster until the vacuum prevailing in the vacuum reservoir has reached a defined switch-on threshold. This occurs mainly when applying or releasing the brake.
It was observed that using electric vacuum pumps for the brake boosting system regulated in this manner may have a negative impact on driving comfort and may irritate the driver when applying the brake.